1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light-emitting element, a light-emitting device, a lighting device, and an electronic device using the quinoxaline derivative.
2. Description of the Related Art
Organic compounds can have a wide variety of structures as compared with inorganic compounds, and have a possibility to provide materials with various functions by appropriate molecular design. Because of these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
As examples of electronic devices using an organic compound as a functional material, there are solar cells, light-emitting elements, organic transistors, and the like. These devices utilize electrical properties and optical properties of the organic compound. In particular, the light-emitting elements have been significantly developed.
It is considered that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the light-emitting layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.
Such a light-emitting element has a lot of problems that depend on the organic materials. In order to solve these problems, improvement of an element structure, development of a material, and the like have been carried out.
As the most basic structure of a light-emitting element, the following structure is known: a hole-transport layer formed of an organic compound with hole-transporting properties and an electron-transport light-emitting layer formed of an organic compound with electron-transporting properties are stacked to form a thin film with a total thickness of about 100 nm, and this thin film is interposed between electrodes (for example, see Reference 1).
When a voltage is applied to the light-emitting element described in Reference 1, light emission can be obtained from an organic compound having light-emitting and electron-transporting properties.
Furthermore, in the light-emitting element described in Reference 1, functions of the thin film are appropriately separated in such a manner that the hole-transport layer transports holes while the electron-transport layer transports electrons and emits light. However, various interactions (for example, exciplex formation) occur at the interface of stacked layers, which may cause a change in emission spectrum or a decrease in emission efficiency.
In order to suppress the change in emission spectrum and the decrease in emission efficiency that are caused by the interaction at the interface, a light-emitting element in which functions of the thin film are further separated has been developed. For example, a light-emitting element having such a structure that a light-emitting layer is sandwiched between a hole-transport layer and an electron-transport layer has been proposed (for example, see Reference 2).